Coral Reefs: Diversity on Display

Whitetip Reef Shark

Despite having a face that looks strangely like that of a disgruntled
weasel, the Whitetip Reef Shark (Triaenodon obesus) is generally
unaggressive toward humans who invade its environment. Although it often
rests in caves during daylight hours, this species is probably the most
commonly-encountered shark of the tropical Pacific. Indeed, for many divers
and snorkellers, the phlegmatic Reef Whitetip is their only ambassador to
sharkdom.

Despite its formal species name, obesus, the Whitetip Reef Shark is
rather slender and delicately built. This body form grants it the ability to
slither, eel-like into crevices in the reef, where this species is an
absolute master at extracting prey. Quiescent during the day, Whitetip Reef
Sharks become active and determined hunters at night. Moving over the reef
face in loosely organized packs, these sharks systematically poke their
blunt heads into each crack and crevice in the reef face in search of prey.
Known prey of the Whitetip Reef Shark includes sleeping diurnal teleosts or
hiding nocturnal creatures such as octopuses, soldierfishes, wrasses, and
trumpetfishes, which are grasped with its small, tricuspid teeth. Using its
ampullae of Lorenzini and uniquely tube-flapped nares, the Whitetip Reef
detects its prey primarily by bioelectrical cues and scent. When a hunting
Whitetip Reef locates a prey animal within a hole in the reef face, it
violently twists and turns to push itself deep into the crevice. Some sharks
actually squirm into a hole in one side of a coral head and exit through an
opening on the other. During these zealous foraging bouts, Whitetip Reef
Sharks have been observed breaking off pieces of coral — sometimes tearing
their skin and fins. Although they are primarily nocturnal, Whitetip Reef
Sharks can and do feed opportunistically by day.

In addition to electrical and olfactory cues, the Whitetip Reef Shark is
also highly responsive to sounds and vibrations. In experiments conducted at
the Marshall Islands and French Polynesia, Whitetip Reefs consistently
responded to recorded sounds of struggling fish, feeding sharks, and even
vocalizations of teleosts. At Eniwetok Atoll, Marshall Islands, artificially
generated sounds were found to be most attractive to Whitetip Reef Sharks
when they combined low frequency (25-100 Hertz) with varied intermittent
pulses every 7.5 to 15 seconds. These sound characteristics correspond well
to the irregular sounds and vibrations generated by a fish struggling on the
end of a spear or on hook and line. In South Pacific areas where
spearfishing is common, Whitetip Reef Sharks respond very rapidly to the
sound of a speargun discharge, typically appearing within seconds. Although
normally quite placid, this inquisitive species can become persistent and
bold when faced with a diver playing or carrying a speared fish — sometimes
dashing in to tear an impaled fish from the spear tip.

Due to the abundance of Whitetip Reef Sharks in coastal areas of the
tropical Pacific and Indian Oceans, day-to-day life of this species is
better known than that of most sharks. Whitetip Reefs are most often
encountered during daylight hours while they rest quietly in underwater
caves throughout much of the tropical Indo-Pacific or in lava tubes of
Hawaiian reefs. But in some locations — such as at Cocos Island and
near-shore waters off the Pacific coast of Costa Rica — this species is
often seen lying stretched out on the sandy bottom, completely exposed in
broad daylight. Sometimes, several of these gregarious sharks are seen lying
side-by-side or even stacked on top of one another, like cord-wood. The
significance of these diurnal al fresco gatherings is not known, but may
have something to do with these sharks being cleaned by small wrasses and at
least one species of goby.

Aspects of the private lives of consenting adult Whitetip Reef Sharks
have been observed. On several occasions at Rangiroa Atoll, Marshall
Islands, two to five mature male Whitetip Reefs were observed following a
mature female, possibly attracted by a pheromone ‘perfume’ released by the
female to announce her readiness to mate. Actual copulation in a pair of
Whitetip Reef Sharks has been photographed in June 1982 at Molokini Crater,
2.5 miles (4 kilometres) southwest of the island of Maui, Hawaii. Mating
took place on the bottom at a depth of 23 feet (7 metres). Both partners’
heads were pressed into the substrate of volcanic rubble and their bodies
were aligned parallel to one another, extending upward into the water column
at an angle of about 45 degrees. The female Whitetip Reef had fresh cuts
above her gill slits and behind her first dorsal fin, presumably from her
suitor’s precopulatory ‘love nips’. The smaller male shark had a tight grasp
on the female’s left pectoral fin as his left clasper was flexed 90 degrees
across his belly and inserted into her vent. The male Whitetip Reef had a
swollen and distended left siphon sac — a subcutaneous organ in male sharks
that stores the seawater used to flush sperm through the clasper groove and
into the reproductive tract of females. After documenting this intimate
moment, the underwater photographer and his diving companions opted to leave
the mating sharks in peace.

Tagging and telemetry studies have revealed much about the home range and
general ecology of the Reef Whitetip Shark. At Mahuta Reef in the Avatoru
Lagoon area of Rangiroa Atoll, 75 Whitetip Reefs were tagged, of which 64
(85%) were resighted. Of these 64 individuals, only 8 (13%) were ever
resighted beyond Mahuta Reef and of these 8 only 2 (25%) were sighted at
other reefs. A year after tagging was completed, about 75% of Whitetip Reefs
at Mahuta still bore tags or tagging scars. Even after 3.3 years, some 40%
of the sharks had remained in the immediate. This strongly suggests that
Whitetip Reef Shark populations are highly stable, with only a trickle of
immigration and emigration each year. Telemetry studies of this species at
Mahuta have demonstrated that typical day-time range is only 0.02 square
miles (0.05 square kilometres) — about the area of Vatican City or about 10
American football fields. At night, however, the same sharks ranged over an
area of about 0.4 square miles (1 square kilometre) — an area 20 times as
large, or about 100 times the area of a union rugby field. These data
suggest that the Whitetip Reef Shark is a definite ‘home-body’, rarely
venturing far from a relatively small area.

The Reef Whitetip is one of the three most common sharks on Indo-Pacific
coral reefs, the other two being the Blackfin Reef Shark (Carcharhinus
melanopterus) and the Grey Reef Shark (Carcharhinus amblyrhynchos). Although
these three sharks are widely distributed over the vast expanse of the
tropical Pacific and Indian Oceans, their relative distribution over the
reef profile is remarkably uniform from place to place. Blackfin Reef
Sharks, especially juveniles, are typically found closest to shore,
inhabiting the turbid lagoon shallows over sandy plains at depth from 0 to
50 feet (0 to 15 metres). Juvenile Grey Reef Sharks are found in the
clearer, deeper waters of the back reef, while adults of this species
typically patrol the reef crest and fore reef from the depth of about 65
feet (20 metres) down to a depth of about 330 feet (100 metres). Both
species are most active at dawn and dusk, accomplishing most of their
feeding during these twilight hours when schooling diurnal fishes are most
vulnerable.

Neatly nestled between the crepuscular Blackfin Reef and Grey Reef
sharks, Whitetip Reefs typically haunt the reef flats and shallower parts of
the fore-reef at depths of 35 to 100 feet (10 to 30 metres). Although it can
extend its range from the intertidal to at least 130 feet (40 metres), the
Whitetip Reef manages to coexist with the Blackfin Reef and Grey Reef Shark
by feeding primarily at night and specializing in extracting prey from
cracks and crevices in the reef face that are all but inaccessible to these
other sharks. By inhabiting different depths and ecological niches, the
slender, weasel-faced Whitetip Reef Shark reduces competition for food
resources with other sharks sharing its habitat.